In the application field of steel, those which require best formability are automobile steel sheets, and electronic panel sheets.
Particularly, in the automobile industry, the discharge of carbon dioxide is more strictly regulated coming recently for alleviating the air pollution. In accordance with this trend, there has been demanded a high strength steel sheet which has a good formability, as well as improving the combustion rate of the fuel, and reducing the weight of the automobile.
Conventionally, as the automobile steel sheet, a extra low carbon steel in which the matrix structure is a ferrite has been used for assuring the formability (U.S. Pat. Nos. 4,950,025, 4,830,686 and 5,078,809).
However, in the case where the extra low carbon steel is used for the automobile steel sheet, although the formability is superior, the tensile strength is lowered to 28-38 kg/mm.sup.2. Consequently the weight of the automobile cannot be reduced, and the safety of the automobile is lowered, thereby jeopardizing the lives of passengers.
The extra low carbon steel having the fenite matrix ferrite can include up to 0.005% of carbon, and the solubility limit for impurities is very low. If carbon and other impurities are added in excess of the solubility limit, then carbides and oxides are formed, with the result that particular textures cannot be developed during cold rolling and annealing processes, thereby degrading the formability.
Thus, in the case of the conventional automobile steel sheet having the fenite matrix, the addition of carbon is reduced to about 0.003%, as well as reducing other impurities to extremely small amounts for enhancing the formability. Consequently, there are accompanied difficulties such that special treatment such as degassing treatment has to be carried out in the steel making process, and that particular textures have to be developed during cold rolling and annealing processes.
Further, a multi-phase steel in which the low strengths of the extra low carbon steel are improved is disclosed in U.S. Pat. No. 4,854,976. In this steel, Si, Mn, P, Al and B are added in large amounts to form a bainite structure and retained austenite structure of less than 8%, thereby increasing the tensile strength to 50-70 kg/mm.sup.2. However, due to the difference of the deformation capabilities between the bainite structure and the retained austenite structure, the formability is lowered, and therefore, this material is limitedly used in automobile parts which do not require a high formability.
Meanwhile, the steel sheet which is used as the external panel of electronic apparatus has to be non-magnetic material which is not influenced by magnetic fields, as well as being high in its strengths and formability. Therefore, austenitic stainless steel is mainly used for this purpose, but this steel contains expensive nickel to about 8%, while its magnetic susceptibility becomes unstable due to strain-induced .alpha.'-martensites during its manufacturing process.
The present inventors have been engaged for many years in studying on how to overcome the disadvantages of the conventional automobile steel sheet and the electronic steel sheet, and have successfully developed an austenitic high manganese steel having superior formability and strengths.
So far, no case has been found in which a high manganese steel is used to attempt providing good formability and high strength.
Currently, the high manganese steel is used in nuclear fusion reactor, in magnetic floating rail for the purpose of preventing electrostatic charges, and as non-magnetic structural material for transformers (Japanese Patent Laying-opening No. Sho-63-35758, 64-17819, 61-288052 and 60-36647). Further, this material is also used as non-magnetic steel for some parts of VTR and electronic audio apparatuses (Japanese Patent Laying-opening No. Sho-62-136557).
However, in this non-magnetic high manganese steel, either Al as an ingredient of the alloy is not added, or it is added up to only 4% for deoxidizing, oxidation resistance, corrosion resistance, solid solution hardening, and grain refinement (Japanese Patent Laying-opening No. Sho-60-36647, 63-35758, and 62-136557)
Meanwhile the alloy of the same composition system which is related to the present invention is disclosed in Korean Patent 29304 (the corresponding U.S. Pat. No. 4,847,046, and Japanese Patent 1,631,935) which is granted to the present inventors).
However, the alloy system which is disclosed in Korean Patent 29304 is considered on its ultra low temperature strength and toughness, and therefore, is for being used in the cryogenic applications. Therefore, it is essentially different from the steel of the present invention which is intended to improve the formability, strengths and weldability.